The present invention relates to an ignition timing control system for an internal combustion engine having an exhaust gas recirculation (EGR) system, and particularly to an EGR/ignition timing control system which provides the desirable vacuum advancing of ignition timing in relation to EGR control so as to adapt ignition timing to EGR operation of the engine.
An EGR system, which recirculates a part of the exhaust gases of an engine from its exhaust system to its intake system, has an EGR passage and an EGR valve incorporated in the EGR passage and is adapted to recirculate a controlled amount of exhaust gases from the exhaust system to the intake system in accordance with operational conditions of the engine. Such an EGR valve is generally a vacuum-operated diaphragm valve which has a diaphragm means and is adapted to be opened when the diaphragm means is supplied with a vacuum greater than a predetermined level, wherein the vacuum supplied to the diaphragm means is taken from a vacuum port provided in the intake system of the engine so as to open to the intake passage at a position which is upstream of the throttle valve incorporated in the intake passage when the throttle valve is fully closed and which is downstream of the throttle valve when it is opened beyond a predetermined opening. The vacuum taken from this vacuum port changes in accordance with the opening of the throttle valve in such a manner that when the throttle valve is fully closed, as when the engine is idling, the vacuum is of zero level; when the throttle valve is gradually opened from its fully closed position so as to traverse the front area of the vacuum port, the vacuum abruptly increases; when the throttle valve is opened somewhat more, the vacuum decreases in turn as the throttle opening increases; when the throttle valve is opened further so as to exceed approximately 50.degree., the vacuum remains constant generally in the range of -5 to -60 mm Hq depending upon the rotational speed of the engine; and when the throttle valve is opened still further, the vacuum again gradually increases as the rotational speed of the engine increases. Therefore, if the EGR valve is designed so as to be opened when its diaphragm means is supplied with a vacuum greater than, for example, -60 mm Hq, the EGR system is automatically controlled in a manner such that the exhaust gas recirculation is not effected when the engine is idling, or is operating at low speed with the throttle valve being fully closed or slightly opened, or when the engine is operating at high load with the throttle valve being widely opened, and such that the exhaust gas recirculation is performed only in the medium load operation between idling, or low speed operation, and high load operation. An EGR valve of the aforementioned kind is generally so constructed as to have a relatively sharp on-off performance at its set vacuum so that if the set vacuum is, for example, -60 mm Hg, the EGR valve begins to open when the vacuum supplied to its diaphragm means reaches -60 mm Hg and is nearly fully opened when the vacuum reaches -70 mm Hg.
When exhaust gas recirculation is performed in an engine, it is desirable that the ignition timing of the engine should be advanced at the rate of about one degree by crank angle per 1% of exhaust gas recirculation in order to compensate for the decrease of combustion speed of the fuel-air mixture due to exhaust gas recirculation. However, in the conventional diaphragm type vacuum advancer which has a diaphragm means and is adapted to advance ignition timing in accordance with the vacuum taken from an advancer port similar to the aforementioned vacuum port of the EGR system and supplied to its diaphragm means, because of the mechanical restrictions imposed thereon with regard to the dimensions of the diaphragm means, the rate of advance of ignition timing available relative to the magnitude of vacuum is limited to about 2.degree. by crank angle (1.degree. by distributor angle) per 10 mm Hg increase of vacuum. Therefore, when the engine is operated in such a condition that the vacuum taken out from the vacuum port is slightly greater than the set vacuum of the EGR valve, the EGR valve is already widely opened so as to effect the designed maximum exhaust gas recirculation, while on the other hand the vacuum advancing of ignition timing is yet small or medium thereby causing the engine to operate with a substantially delayed ignition timing in view of the influence of exhaust gas recirculation and therefore to operate with a relatively lower output power and a relatively higher fuel consumption rate.